1. Field of the Invention
This invention relates to a semiconductor device having a circuit comprising thin film transistors (hereinafter called xe2x80x9cTFTxe2x80x9d) on a substrate having an insulation surface, and a fabrication method thereof. More particularly, the present invention provides a technology that will be utilized advantageously for an electro-optical device typified by a liquid crystal display device having a pixel unit and a driving circuit disposed round the pixel unit, and for an electronic appliance having such an electro-optical device mounted thereto. Incidentally the term xe2x80x9csemiconductor devicexe2x80x9d used herein represents those devices which operate by utilizing semiconductor characteristics, and embraces within its scope the electro-optical devices as well as the electronic appliances having the electro-optical device mounted thereto that are described above.
2. Description of the Related Art
A technology that uses TFTs for constituting switching devices and functional circuits has been developed in the electro-optical device typified by an active matrix type liquid crystal display device. In the TFT, a semiconductor film is grown on a substrate such as a sheet of glass by a vapor phase growing method, and the semiconductor film is used as an active layer. Silicon or a material consisting of silicon as the principal component such as silicon-germanium has been used appropriately for the semiconductor film. An amorphous silicon film and a crystalline silicon film represented by a polycrystalline silicon film can be obtained depending on the formation method of the silicon semiconductor film.
The TFT using the amorphous silicon film for the active layer cannot essentially acquire field effect mobility of greater than several Cm2/Vsec because of its electro-physical factors resulting from the amorphous structure, and so forth. Therefore, though it can be used as a switching device (pixel TFT) for driving a liquid crystal disposed at each pixel of a pixel unit in an active matrix type liquid crystal device, the amorphous silicon film cannot form a driving circuit for effecting image display. For this reason, a technology of packaging a driver IC, etc, by using aTAB (Tape Automated Bonding) system or a COG (Chip on Glass) system has been employed.
On the other hand, the TFT using the crystalline silicon film for the active layer can acquire high field effect mobility and can form various functional circuits on the same glass substrate. The crystalline silicon film makes it possible to fabricate a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like, each comprising a CMOS circuit including n channel TFTs and p channel TFTs in the driving circuit besides the pixel TFTs. To achieve the reduction of weight and thickness in the liquid crystal display device on the basis of such a technology, it has proved clear that the TFT using the crystalline semiconductor film, that can form integrally the driving circuit on the same substrate besides the pixel unit, for the active layer is suitable.
From the aspect of performance of the TFT, the active layer using the crystalline silicon film is superior. To form the TFT that can cope with various circuits besides the pixel TFTs, however, the fabrication steps become complicated and the number of process steps increases. The increase of the number of process steps in turn results in the increase of the production cost and lowers also the production yield.
For example, the operating condition of the circuits are not always the same for the pixel TFT and the TFT of the driving circuit. Therefore, the characteristics required for each TFT are different. The pixel TFT comprises an n channel TFT, applies the voltage and drives a liquid crystal as a switching device. Since the liquid crystal is driven by the alternating current, a system called xe2x80x9cframe inversion drivingxe2x80x9d has been used widely. To limit power consumption to a low leveling this system. one of the characteristics required for the pixel TFT is to restrict an OFF current value (a drain current that flows when the TFT is under the OFF operation) to a sufficiently low level. On the other hand, a high driving voltage is applied to a buffer circuit of a control circuit. Therefore, the withstand voltage must be increased less the TFT is not broken even when a high voltage is applied thereto. To improve a current driving capacity, a sufficient ON current value (the drain current that flows when the TFT is under the ON operation) must be secured.
A lightly doped drain (LDD) structure is known as a TFT structure for reducing the OFF current value. This structure disposes an impurity region, to which an impurity element is added in a concentration lower than that of a source or drain region, between a channel formation region and the source or drain region that is formed by adding an impurity element in a high concentration. This impurity region is called the xe2x80x9cLDD regionxe2x80x9d.
As described above, the required characteristics are not always the same between the pixel TFT and the TFT used for the driving circuit such as the shift register circuit or the buffer circuit. For example, a large back-bias (a negative voltage in the case of the n channel TFT) to the gate of the pixel TFT, but the TFT of the driving circuit does not basically operate under the back-bias state. As to the operation speed, too, the operation speed of the pixel TFT may not be higher than {fraction (1/100)} of that of the TFT of the control circuit.
To stabilize the operation of these circuit fabricated by using the n-and p-channel TFTs, the threshold voltage and sub-threshold coefficient (S value) of the TFTs must be kept within predetermined ranges. For this purpose, the TFT must be examined from the aspects of both structure and material.
The present invention contemplates to provide a technology that solves these problems. In electro-optical devices typified by an active matrix liquid crystal device fabricated by using TFTS, the present invention is directed to improve the operation characteristics and reliability of the semiconductor devices by optimizing the structures of the TFTs employed in various circuits in accordance with the functions of the respective circuits, to lower power consumption, and the production cost by reducing the number of process steps, and to improve the production yield.
To accomplish the reduction of the production cost and the-improvement of the production yield by reducing the number of process steps, the number of photo-masks used for the fabrication of the TFT must be reduced. In photolithography, the photo-mask is used for forming a resist pattern as the mask for the etching process on the substrate. Therefore, when one photo-mask is used, additional process steps such as peeling, washing, drying, etc, of the resist are necessary before and after the etching step in addition to the process steps of the film formation and etching. In the photolithography step, too, complicated process steps such as the application of the resist, pre-baking, exposure, development, post-baking, etc, :are necessary.
To accomplish the object described above, the present invention provides a semiconductor device having, on the same substrate, pixel TFTs disposed in a pixel unit and a driving circuit including p channel type TFTs and n channel type TFTs and disposed round the pixel unit, wherein the p channel type TFT of the driving circuit has a channel formation region and a p type impurity region having a third concentration, for forming a source region or a drain region; the n channel type TFT of the driving circuit and the pixel TFT each have a channel formation region, an n type impurity region having a first concentration, disposed in contact with the channel formation region and forming an LDD region, and an n type impurity region for forming a source region or a drain region, having a second concentration and disposed outside the n type impurity region having the first concentration; andeachpixel electrode disposed in the pixel unit and having a light reflecting surface is formed on an inter-layer insulation film made of an organic insulating material, and is connected to the pixel TFT through a hole bored in at least a protective insulation film made of an inorganic insulating material and disposed above a gate electrode of said pixel TFT and said inter-layer insulation film formed on the insulation film in close contact therewith.
Another construction of the present invention provides a semiconductor device having, on the same substrate, pixel TFTs disposed in a pixel unit and a driving circuit including p channel type TFTs and n channel type TFTs and disposed round the pixel unit, wherein: the p channel type TFT of the driving circuit has a channel formation region and a p type impurity region having a third concentration, for forming a source region or a drain region; the n channel type TFT of the driving circuit and the pixel TFT each have a channel formation region, an n type impurity region having a first concentration, disposed in contact with the channel formation region and forming an LDD region, and an n type impurity region for forming a source region or a drain region, having a second concentration and disposed outside the n type impurity region having the first concentration; and each pixel electrode disposed in the pixel unit and having a light reflecting surface is formed on an inter-layer insulation film made of an organic insulating material, and is connected to an electrically conductive lead wire connected to the pixel TFT, through a hole bored in at least a protective insulation film made of an inorganic insulating material and disposed above a gate electrode of said pixel TFT and said inter-layer insulation film formed on the protective insulation film in close contact therewith.
Another construction of the present invention provides a semiconductor device having a liquid crystal sandwiched between a pair of substrates, wherein, in one of said substrates having pixel TFT of a pixel unit and p channel type TFTs and n channel type TFTs of a driving circuit; the p channel type TFT of the driving circuit has a channel formation region and a p type impurity region having a third concentration, for forming a source region or a drain region; the n channel type TFT of said driving circuit and the pixel TFT each have a channel formation region, an n type impurity region having a first concentration, disposed in contact with the channel formation region and forming an LDD region, and an n type impurity region for forming a source region or a drain region, having a second concentration and disposed outside said n type impurity region having the first concentration; each pixel electrode disposed in the pixel unit and having a light reflecting surface is formed on an inter-layer insulation film made of an organic insulating material and is connected to said pixel TFT through a hole bored in at least a protective insulation film made of an inorganic insulating material and disposed above a gate electrode of the pixel TFT and said inter-layer insulation film formed on the protective insulation film in close contact therewith; and this one substrate is bonded to the other substrate having a transparent conductor film formed thereon through at least one columnar spacer formed in superposition with the hole.
Another construction of the present invention provides a semiconductor device having a liquid crystal sandwiched between a pair of substrates, wherein, in one of the substrates having pixel TFTs of a pixel unit and p channel type TFTs and n channel type TFTs of a driving circuit; the p channel type TFT of the driving circuit has a channel formation region and a p type impurity region having a third concentration, for forming a source region or a drain region; the n channel type TFT of the driving circuit and the pixel TFT each have a channel formation region, an n type impurity region having a first concentration, disposed in contact with the channel formation region and forming an LDD region, and an n type impurity region for forming a source region or a drain region, having a second concentration and disposed outside the n type impurity region having the first concentration; each pixel electrode disposed in the pixel unit and having a light transmitting property is formed on an inter-layer insulation film made of an organic insulating material and is connected to a conductive metal lead wire connected to the pixel TFT through a hole bored in at least a protective insulation film made of an inorganic insulating material and disposed above a gate electrode and said inter-layer insulation film formed on the protective insulation film in close contact therewith; and
this one substrate is bonded to the other of the substrates having a transparent conductor formed thereon through at least one columnar spacer formed in superposition with the hole.
In the present invention, the p channel type TFT of the driving circuit has an offset region formed between the channel formation region and the p type impurity region having the third concentration, for forming the source region or the drain region.
In a method of fabricating a semiconductor device having, on the same substrate, pixel TFTs disposed in a pixel unit and a driving circuit including p channel type TFTs and n channel type TFTs and disposed round said pixel unit, a method of fabricating a semiconductor device according to the present invention comprises the steps of: forming an underlying film on the substrate; forming a plurality of island-like semiconductor layers on the underlying film; forming an n type impurity region having a first concentration, for forming an LDD region of the n channel type TFT of the driving circuit and the pixel TFT in a selected region of the island-like semiconductor layer; forming an n type impurity region having a second concentration, for forming a source region or a drain region outside the n type impurity region having the first concentration; forming a p type impurity region having a third concentration, for forming a source region or a drain region of the p channel type TFT of the, driving circuit in a selected region of the island-like semiconductor layer; forming a protective insulation film formed of an inorganic insulating material above the n channel type TFT of the driving circuit, the pixel TFT and the p channel type TFT; forming an inter-layer insulation film formed of an organic insulating material in close contact with the protective insulation film; and forming a pixel electrode having a light reflecting surface and connected to the pixel TFT on the inter-layer insulating film.
In a method of fabricating a semiconductor device having, on the same substrate, pixel TFTs disposed in a pixel unit and a driving circuit including p channel type TFTs and n channel type TFTs and disposed round the pixel unit, the present invention provides a method of fabricating a semiconductor device that comprises the steps of: forming an underlying film on the substrate; forming a plurality of island-like semiconductor layers on the underlying film; forming an n type impurity region having a first concentration, for forming an LDD region of the n channel type TFT of the driving circuit and the pixel TFT in a selected region of the island- like semiconductor layer; forming a high concentration n type impurity region for forming a source region or a drain region outside the n type impurity region having the first concentration; forming a p type impurity region having a third concentration, for forming a source region or a drain region of the p channel type TFT of the driving circuit in a selected region of the island- like semiconductor layers; forming a protective insulation film formed of an inorganic insulating material above the n channel type TFT of the driving circuit, the pixel TFT and the p channel type TFT; forming an inter-layer insulation film formed of an organic insulating material in close contact with the protective insulation film; forming a conductive metal lead wires to be connected to the pixel TFT; and forming a pixel electrode comprising a transparent conductor film to be connected to the conductive metal lead wires, on the inter-layer insulation film.
In a method of fabricating a semiconductor device having a liquid crystal sandwiched between a pair of substrates, the present invention provides a method of fabricating a semiconductor device that comprises the following steps for one of substrates including pixel TFTs disposed in a pixel unit and a driving circuit having p channel type TFTs and n channel type TFTs round the pixel unit: forming an underlying film on the substrate; forming a plurality of island-like semiconductor layer on the underlying film; forming an n type impurity region having a first concentration, for forming an LDD region of the n channel type TFT of the driving circuit and the pixel TFT, in a selected region of the island-like semiconductor layers; forming an n type impurity region having a second concentration, for forming a source region or a drain region outside the n type impurity region having the first concentration; forming a p type impurity region having a third concentration, for forming a source region or a drain region of the p channel type TFT of said driving circuit, in a selected region of the island-like semiconductor layer; forming a protective insulation film formed of an inorganic insulating material above the gate electrodes of the n channel type TFT of the driving circuit, the pixel TFT and the p channel type TFT; forming an inter-layer insulating film formed of an organic insulating material in close contact with the protective insulation film; and forming a pixel electrode having a light reflecting surface and to be connected to the pixel TFT through a hole bored in the inter-layer insulation film and in the protective insulation film, on the inter-layer insulation film: and comprises, as for the other of the substrates, the step of forming at least a transparent conductor film; the method further comprising the step of bonding the one substrate to the other substrate through at least one columnar spacer formed in superposition with the hole.
In a method of fabricating a semiconductor device having a liquid crystal sandwiched between a pair of substrates, the present invention provides a method of fabricating a semiconductor device that comprises the following steps for one of substrates having pixel TFTs disposed in a pixel unit and a driving circuit having p channel type TFTs and n channel type TFTs and disposed round the pixel unit; forming an underlying film on the substrate; forming a plurality of island-like semiconductor layers on the underlying film; forming an n type impurity region having a first concentration, for forming an LDD region of the n channel type TFT and the pixel TFT, in; a selected region of the island-like semiconductor layer; forming an n type impurity region having a second concentration, for forming a source region or a drain region outside the n type impurity region having the first concentration; forming a p type impurity region having a third concentration, for forming a source region or a drain region of the p channel type TFT of the driving circuit, in a selected region of the island-like semiconductor layer; forming a protective insulation film formed of an inorganic insulating material above the n channel type TFT of the driving circuit, the pixel TFT and the p channel type TFT; forming an inter-layer insulation film formed of an organic insulating material in close contact with the protective insulation film; forming a conductive metal lead wire connected to the pixel TFT, through a hole bored in the inter-layer insulation film and the protective insulation film; and forming a pixel electrode comprising a transparent conductor film to be connected to the metal lead wire, on the inter-layer insulation film; and comprises, as for the other of the substrates, the step of forming at least a transparent conductor film on the other of the substrates; the method further comprising the step of bonding the one substrate to the other substrate through at least one columnar spacer formed in superposition with the hole.
In the method of fabricating a semiconductor device described above, as for the p channel type TFT of the driving circuit, the step of forming a p type impurity region having a third concentration, for forming a source region or a drain region of the p channel type TFT can be conducted in a selected region of the island-like semiconductor layers after the step of forming the protective insulation film formed of an inorganic insulating material, on the gate electrode of the p channel type TFT, and an offset region can be formed between the channel formation region of the p channel type TFT and the p type impurity region having the third concentration, for forming the source region or the drain region.